James Webb Space Telescope Feed Post

Arxiv: The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST Published: 9/24/2024 9:00:02 PM Updated: 9/24/2024 9:00:02 PM

Paper abstract: The characterization of young planets (< 300 Myr) is pivotal forunderstanding planet formation and evolution. We present the 3-5\mumtransmission spectrum of the 17 Myr, Jupiter-size (R ~10R_{\oplus})planet, HIP 67522 b, observed with JWST/NIRSpec/G395H. To check for spotcontamination, we obtain a simultaneous g-band transit with SOAR. Thespectrum exhibits absorption features 30-50% deeper than the overall depth, farlarger than expected from an equivalent mature planet, and suggests that HIP67522 b's mass is <20 M_{\oplus} irrespective of cloud cover and stellarcontamination. A Bayesian retrieval analysis returns a mass constraint of13.8\pm1.0M_{\oplus}. This challenges the previous classification of HIP67522 b as a hot Jupiter and instead, positions it as a precursor to the morecommon sub-Neptunes. With a density of <0.10g/cm^{3}, HIP 67522 b is one ofthe lowest density planets known. We find strong absorption from H_{2}O andCO_{2} (\ge7\sigma), a modest detection of CO (3.5\sigma), and weakdetections of H_2S and SO_2 (~eq2\sigma). Comparisons withradiative-convective equilibrium models suggest supersolar atmosphericmetallicities and solar-to-subsolar C/O ratios, with photochemistry furtherconstraining the inferred atmospheric metallicity to 3\times10 Solar due tothe amplitude of the SO_2 feature. These results point to the formation ofHIP 67522 b beyond the water snowline, where its envelope was polluted by icypebbles and planetesimals. The planet is likely experiencing substantial massloss (0.01-0.03 M_{\oplus} Myr^{-1}), sufficient for envelope destructionwithin a Gyr. This highlights the dramatic evolution occurring within the first100 Myr of its existence.